With the development of radio and space technologies, several satellites based navigation systems have already been built and more will be in use in the near future. One example of such satellites based navigation systems is Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PRN) code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During the acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
Coherent integration and noncoherent integration are two commonly used integration methods to acquire GPS signals. Coherent integration provides better signal gain at the cost of larger computational load, for equal integration times.
The power associated with noncoherent integration with one millisecond correlation is
  Power  =            ∑              n        =        0                    N        -        1              ⁢          (                                    I            ⁡                          (              n              )                                2                +                              Q            ⁡                          (              n              )                                2                    )      and the power associated with coherent integration is
  Power  =                    (                              ∑                          n              =              0                                      N              -              1                                ⁢                      I            ⁡                          (              n              )                                      )            2        +                  (                              ∑                          n              =              0                                      N              -              1                                ⁢                      Q            ⁡                          (              n              )                                      )            2      where I(n) and Q(n) denote the in-phase and quadra-phase parts of one-millisecond correlation values from the baseband section at interval n, and N denotes the desired number of one-millisecond integration intervals.
The use of coherent integration is desired in weak signal acquisition, reacquisition and tracking. However to achieve a long coherent integration two conditions need to be met. These conditions are that the residual frequency must be very small and signal encoded data bits must be wiped off if the integration time is beyond the navigation data bit interval. Usually the residual frequency is made small by employing a large number of frequency bins. On the other hand once the integration period is more than 20 milliseconds, which is the navigation data bit duration, the modulated data need to be removed from the signal before coherent integration. A prior knowledge of the data bits is needed for this purpose and this may be based upon the structure of the navigation message. A brief explanation on the structure of the message which may help in predicting the data is given below.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second. This data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames, frames and super-frames. A sub-frame consists of 300 bits of data and is transmitted for 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A super-frame consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is always the same and is known as TLM word and first eight bits of this TLM word are preamble bits used for frame synchronization. A Barker sequence is used as the preamble because of its excellent correlation properties. The other bits of this first word contain 14 telemetry bits and is not used in the position computation for civilian applications. The next two reserved bits are always ‘1’s. The remaining 6 bits in the first word are parity check bits. The second word of any frame is the HOW (Hand Over Word) word and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity with the last two bits of parity always being ‘0’s. These two ‘0’s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contains clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemeris are used to precisely determine the position of the GPS satellites. These ephemeris are uploaded every two hours and are valid for four hours to six hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and almanac of some satellites. These almanacs are similar to the ephemeris but give a less accurate position of the satellites. The 3 to 10 words of the sub-frame 5 contain only the almanacs of different satellites in different frames.
The superframe contains twenty five frames. The contents of the sub-frame 1, 2 and 3 repeat in every frame of a superframe except the TOW and occasional change of ephemeris every two hours. Thus the ephemeris of a particular satellite signal contains only the ephemeris of that satellite repeating in every frame. However, almanacs of different satellites are broadcast in-turn in different frames of the navigation data signal of a given satellite. Thus a total of 25 frames transmit the almanacs of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanacs are included in some of the sub-frame 4.
Thus from the above, it is possible to predict some of the navigation data bits and use it to wipe off the modulated navigation data from the received signal. The data prediction can be done easily as most of the data bits remain unchanged over a length of time. The ephemeris, for example, e remains the same over time length of two hours. The almanac remains the same for several days. The synchronizing word in TLM never changes while HOW word can be easily predicted. These facts may be used to predict the data and wipe off the data from the received signal thus ensuring a long coherent integration extending over several data bits. There exists some prior art wherein the data bits are predicted to carry out a long integration. Published U.S. Patent Application 2003/0152134 predicts data under weak signal reception condition. Published U.S. Patent Application 2002/0049536 assigned to Qualcomm performs data wipe off by predicting the HOW word. This prediction is done by changing the TOW value as required and the associated parity. U.S. Pat. No. 6,611,756 assigned to Lucent similarly predicts not only the HOW but also the TLM and other data bits. U.S. Pat. No. 6,252,545 assigned to LUCENT discloses methods of long integration to acquire and track weak signals while U.S. Pat. No. 6,965,760 teaches a dynamic integration technique based on the received signal strength. All of these methods carry out integration over as much of the data bits as possible. However, it is desirable for a handheld navigation receiver to save as much power as possible while employing a long integration under weak signal conditions.